How to close an area?
Ok, I found one weird way:

I can split the bottom area vertically as well and then align the split borders of the top and bottom area pairs:



This alignment is easily possible because the split borders snap to a grid.

As soon as they are aligned, the context menu of the border between any top and bottom area gets a new item "Join areas":



After joining the left pair and the right pair, the bottom area is then gone.